Air cycle environmental control systems that condition air are well-known for the cooling and/or pressurization of heat loads such as passenger cabins and equipment compartments in both commercial and military aircraft. These systems are popular for a number of reasons, including the substantial amount of cooling available from air cycle systems of relatively modest size, and the adaptability of such systems to gas turbine engine powered vehicles such as aircraft and military land vehicles such as battle tanks.
In a typical installation, compressed ambient air provided by either the compressor section of a main engine or the compressor section of an auxiliary power unit, or both, is expanded in an air cycle turbo machine to provide a cool, fresh air supply for one or more cabins and/or equipment compartments of a vehicle.
One known air cycle environmental control system that utilizes a compressor (36) and a fan (22) driven by a pair of turbines (24) and (26) via a common shaft (23) is disclosed in U.S. Pat. No. 5,086,622, issued on Feb. 11, 1992 to John L. Warner and assigned to the Assignee of the present application, the entire disclosure of which is incorporated herein by reference. In Warner's system, compressed air is cooled in a primary heat exchanger (16), compressed in the compressor (36), and then cooled again in a secondary heat exchanger (40). The fan (22) drives a cooling ambient air flow through the primary and secondary heat exchangers (16) and (40). After passing through the primary and secondary heat exchangers (16) and (40), the compressed air is then further cooled in a condenser (46) that dehumidifies the compressed air by condensing water-vapor from the compressed air. The dehumidified compressed air is then directed through the turbine (24) where it is expanded to provide power to the shaft (23) and to cool the compressed air so that it may be used as the coolant in the condenser (46). The compressed air is then further expanded through the turbine (26) to power the shaft (23) and to cool the compressed air so that it may be supplied to a cabin (62). While this system is more than satisfactory for a number of applications, such as for large commercial aircraft, it may not be the optimum system for all applications, such as for small regional aircraft.
U.S. Pat. No. 5,887,445 issued Mar. 30, 1999 to Murry et al. discloses another air cycle environmental control system (10) that utilizes two air cycle machines (39) and (40) that rotate independent of each other. The air cycle machine (39) includes a compressor (16) that is driven by a high pressure turbine (24), and the air cycle machine (40) includes a fan (32) that is driven by a low pressure turbine (28) to supplement a ram air flow through primary and secondary heat exchangers (12,13). The system (10) also includes a reheater (19), condenser (26), and water extractor (21) that dehumidify the bleed air flow to the high pressure turbine (24). While the system (10) of Murry et al. may satisfactorily perform its intended function, for certain applications operation of the system (10) with an optimum power split between the turbines (24) and (28) may require that, under normal operating conditions, the bleed air be expanded through the high pressure turbine (24) to a point where the condensed water in the condenser (26) freezes, thereby creating icing in the condenser (26). This may require that the condenser (26) be designed to accommodate such icing, which can increase the size, weight, cost, and complexity of the condenser (26).